1. Field of the Invention
This invention relates to automatic belt tensioner for adjusting the tension of an engine timing belt in apparatus such as automobiles, and more particularly to automatic belt tensioners of the self-contained type in which a fluid is sealed within the assembly without provision for its replacement from any external source during operation.
2. Description of the Prior Art
Various types of automatic belt tensioners for preventing the slackening of an engine timing belt for automobiles have been proposed. One example of a prior art automobile belt tensioner is disclosed in the Japanese patent application laid-open publication No. 59(1984)-126144 as shown in FIG. 4 attached hereto. In FIG. 4, an automatic belt tensioner comprises a body 1 in which a plunger 2 is slidably mounted so as to form a large diameter oil chamber 4 in which a fluid is sealed by means of a seal member 3. The plunger 2 is continuously biased in the direction of the oil chamber 4 by a spring 7 interposed between a snap ring 5 and a stepped portion 6 of the plunger 2.
The plunger 2 is provided with a hole 8 in which a piston 9 is slidably disposed so as to form a small diameter oil chamber 11 communicating with the large diameter oil chamber 4 through a small diameter passage 10. The small diameter oil chamber 11 is sealed from the outside by means of a seal member 12. Secured to an open portion of the hole 8 is a snap ring 13 which may come into contact with a stepped portion 14 of the piston 9 in order to prevent the piston 9 from coming out. A head portion 9a of the piston 9 protrudes outwardly from the body 1. An end surface 9b functions as a pushing surface of the automatic belt tensioner.
When an engine temperature rises from a normal temperature to a high temperature, a thermal expansion is generated between a cam and a crank whereby the tension of a belt a shown in FIG. 3 is increased. At this time, since the load applied to the piston 9 is increased, the inner oil pressure of the small diameter oil chamber 11 is also increased. Therefore, the oil flows from the small diameter oil chamber 1 into the large diameter oil chamber 4 through a small diameter passage 10, with the result that the piston 9 is moved downward in the side of the body 1 and that the increase of the belt tension is absorbed. In FIG. 3 showing a general automatic belt tensioner, an automatic belt tensioner C installed in a bracket b pushes the belt a through an idler pulley d.
On the contrary, when the engine temperature falls from the high temperature to the normal temperature, the tension of the belt is decreased whereby the load applied to the piston 9 is decreased. Therefore, due to the inner oil pressure of the small diameter oil chamber 11, the piston 9 will protrude from the body 1 and push the belt, whereby the decrease amount of the belt tension can be absorbed. Thus, an appropriate tension of the belt can be maintained.
In the above prior art automatic belt tensioner, however, the piston receives the load of the belt and faces at the bottom end thereof the small diameter oil chamber 11. When the piston 9 is pushed down, a high oil pressure is generated in the small diameter oil chamber 11. Since the high oil pressure acts on the seal member 12 through a gap between the piston 9 and the plunger 2, a bad influence is produced that the seal member 12 is deformed. Furthermore, the high oil pressure in the small diameter oil chamber 11 is transmitted to the large diameter oil chamber 4 through the small diameter passage 10. Since the large diameter oil chamber 4 receives the biasing force of the spring 7, the high oil pressure in the large diameter oil chamber 4 has a bad influence on the seal member 3 through the gap between the plunger 2 and the body 1. Therefore, the sealing function of the seal members 3 and 12 will be deteriorated and the oil will leak out of the seal members 3 and 12.